Re: [PATCH V1 13/16] xen/ioreq: Make x86's invalidate qemu mapcache handling common

[Oleksandr <olekstysh@xxxxxxxxx>]

Hi Jan


On 25.09.20 16:05, Oleksandr wrote:
> On 25.09.20 10:03, Jan Beulich wrote:
>
> Hi Jan.
>
> > On 24.09.2020 18:45, Oleksandr wrote:
> > > On 24.09.20 14:16, Jan Beulich wrote:
> > >
> > > Hi Jan
> > >
> > > > On 22.09.2020 21:32, Oleksandr wrote:
> > > > > On 16.09.20 11:50, Jan Beulich wrote:
> > > > > > On 10.09.2020 22:22, Oleksandr Tyshchenko wrote:
> > > > > > > --- a/xen/common/memory.c
> > > > > > > +++ b/xen/common/memory.c
> > > > > > > @@ -1651,6 +1651,11 @@ long do_memory_op(unsigned long cmd, 
> > > > > > > XEN_GUEST_HANDLE_PARAM(void) arg)
> > > > > > >             break;
> > > > > > >         }
> > > > > > >     +#ifdef CONFIG_IOREQ_SERVER
> > > > > > > +    if ( op == XENMEM_decrease_reservation )
> > > > > > > +        curr_d->qemu_mapcache_invalidate = true;
> > > > > > > +#endif
> > > > > > I don't see why you put this right into decrease_reservation(). This
> > > > > > isn't just to avoid the extra conditional, but first and foremost to
> > > > > > avoid bypassing the earlier return from the function (in the case of
> > > > > > preemption). In the context of this I wonder whether the ordering of
> > > > > > operations in hvm_hypercall() is actually correct.
> > > > > Good point, indeed we may return earlier in case of preemption, I 
> > > > > missed
> > > > > that.
> > > > > Will move it to decrease_reservation(). But, we may return even 
> > > > > earlier
> > > > > in case of error...
> > > > > Now I am wondering should we move it to the very beginning of command
> > > > > processing or not?
> > > > In _this_ series I'd strongly recommend you keep things working as
> > > > they are. If independently you think you've found a reason to
> > > > re-order certain operations, then feel free to send a patch with
> > > > suitable justification.
> > > Of course, I will try to retain current behavior.
> > >
> > >
> > > > > > I'm also unconvinced curr_d is the right domain in all cases here;
> > > > > > while this may be a pre-existing issue in principle, I'm afraid it
> > > > > > gets more pronounced by the logic getting moved to common code.
> > > > > Sorry I didn't get your concern here.
> > > > Well, you need to be concerned whose qemu_mapcache_invalidate flag
> > > > you set.
> > > May I ask, in what cases the *curr_d* is the right domain?
> > When a domain does a decrease-reservation on itself. I thought
> > that's obvious. But perhaps your question was rather meant a to
> > whether a->domain ever is _not_ the right one?
> No, my question was about *curr_d*. I saw your answer
> > I'm also unconvinced curr_d is the right domain in all cases here;
> and just wanted to clarify these cases. Sorry if I was unclear.
>
>
> > > We need to make sure that domain is using IOREQ server(s) at least.
> > > Hopefully, we have a helper for this
> > > which is hvm_domain_has_ioreq_server(). Please clarify, anything else I
> > > should taking care of?
> > Nothing I can recall / think of right now, except that the change
> > may want to come under a different title and with a different
> > description.As indicated, I don't think this is correct for PVH
> > Dom0 issuing the request against a HVM DomU, and addressing this
> > will likely want this moved out of hvm_memory_op() anyway. Of
> > course an option is to split this into two patches - the proposed
> > bug fix (perhaps wanting backporting) and then the moving of the
> > field out of arch.hvm. If you feel uneasy about the bug fix part,
> > let me know and I (or maybe Roger) will see to put together a
> > patch.
> Thank you for the clarification.
>
> Yes, it would be really nice if you (or maybe Roger) could create a 
> patch for the bug fix part.

Thank you for your patch [1].

If I got it correctly there won't be a suitable common place where to 
set qemu_mapcache_invalidate flag anymore
as XENMEM_decrease_reservation is not a single place we need to make a 
decision whether to set it
By principle of analogy, on Arm we probably want to do so in 
guest_physmap_remove_page (or maybe better in p2m_remove_mapping).
Julien, what do you think?


I will modify current patch to not alter the common code.


[1] https://patchwork.kernel.org/patch/11803383/

--

Regards,

Oleksandr Tyshchenko